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Why do onions make us cry?

You know that familiar feeling: tingly, stinging eyes, followed by tears. Why would the humble onion, a key ingredient in your next culinary creation, cause you such discomfort?

Onions act in self-defense, and tears are the consequence.

For plants and animals alike, life is all about survival and reproduction. Onions are no exception. To stop greedy predators from eating the bulbs, onions release volatile chemicals that cause our eyes to well up.

Onions are perennial plants and part of the Allium family. This means that in the first year of their life cycle, they form a tasty bulb that serves as an energy store. The following year, the plant produces a flower and seeds, allowing it to reproduce.

Sophisticated chemistry is needed to defend the onion from attack, and scientists have recently identified how the enzyme responsible for this process works.

Tear factor

The chemical at the heart of our discomfort is called propanethial S-oxide, which is also known as lachrymatory factor (LF). The technical term for our tear glands is "lacrimal glands," and LF is a chemical that causes tears.

Only three other molecules with similar tear-inducing properties have been found to date, and they are all produced by plants.

Marcin Golczak, Ph.D. - an assistant professor in the Department of Pharmacology at Case Western Reserve University in Cleveland, OH - and colleagues have recently discovered the structure of the enzyme that produces LF in onions: lachrymatory factor synthase (LF synthase).

When we start chopping, the cells inside the onion are broken up. As a consequence, an enzyme called allinase is released, which produces the chemicals that are subsequently broken down into flavor molecules. These give onions their characteristic taste.

Some of the chemicals involved in this reaction are turned into LF by LF synthase. When LF comes into contact with the front of the eye, or the cornea, nerve endings located here signal to the brain that an irritant has arrived on the scene. This, in turn, leads to signaling back from the brain to the tear glands.

Tears and blinking are subsequently initiated to remove the pesky irritant from the cornea.

The elusive enzyme

Understanding the structure and function of enzymes is a complex science. Although LF synthase was identified as early as 2002, until now, no one has been able to show how it actually works.

Prof. Golczak and his colleagues turned solutions of the enzyme into microscopic crystals, which they were then able to visualize using X-rays. This allowed them to determine the 3-D structure of the enzyme and identify the small pocket in which the chemical conversion of LF takes place.

Detailed knowledge of this chemical process fills a fundamental gap in knowledge and gives scientists a better understanding of the biochemical potential of onions.

Harnessing health benefits

Onions have a wealth of health benefits. Studies suggest a link between the chemicals in onions and the inhibition of cancer growth, as well as improvement of sleep and mood.

As they are also a very good source of dietary vitamin C, which is a building block of collagen, onions may well play a role in hair and skin health.

To save us from the discomfort of tears, two groups of scientists have already been working on producing tear-free onions. One of these new onions has been shown to inhibit platelet aggregation, which is associated with cardiovascular disease.

The next time you are getting your chopping board ready to whip up a meal full of healthful onions, think of the amazing biochemical processes that will unfold in front of your teary eyes.

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